1. Field of the Invention
The present invention relates to a television signal processing apparatus conforming to a wide television signal transmission system possessing a larger aspect ratio than 4:3 and capable of transmitting pictures of high quality, while maintaining compatibility with the existing television broadcasting system with the aspect ratio of 4:3 such as NTSC system and
system.
2. Description of the Related Prior Art
Among the wide television signal transmission systems keeping compatibility with the existing television broadcasting system, a system using the so-called letter box method is known. This method is to transmit a video signal in aspect ratio of 16:9 by using, for example in the case of the NTSC system, about 180 lines of the effective scanning lines (about 240 lines per field). By using the remaining scanning lines (about 60 lines per field), it is proposed to transmit some additional information signal. As such additional information, for example, horizontal high frequency component, vertical high frequency component, and vertical-temporal high frequency component of image are considered. Since these signals have both positive and negative polarities, a certain offset value is added at the transmission side so as to settle within the signal level transmittable by the existing television broadcasting system (disclosed in, for example, WO 90/14732).
When processing thus transmitted additional information signal at the reception side, it is necessary to remove the offset value added to the additional information signal to reproduce the original additional information signal. It is hence considered to transmit an offset reference value as shown in, e.g., ITEJ Technical Report Vol. 14, No. 20, pp. 19-24, BCS '90-22, March, 1990, and ITEJ Technical Report Vol. 14, No. 28, pp. 19-24, CE '90-22, BCS '90-28, May, 1990. At the reception side, after detecting this offset reference value, the value is retained until a next offset reference value is received, and is subtracted from the transmitted signal to reproduce the original additional information signal.
Generally, in a television receiver, the DC level of signal is fixed by the clamper, but the DC level of the output signal varies depending on the performance of the clamper. From such signal with varying DC level, when the offset reference value kept at a constant value is subtracted, an unnecessary offset is left in the additional information signal.